Using the time-of-flight principle of high frequency sound waves, it is possible to accurately measure distances within an aqueous medium. High frequency sound, or ultrasound, is defined as vibrational energy that ranges in frequency from 100 kHz to 10 MHz. The device used to obtain three dimensional measurements is known as a sonomicrometer. Typically, a sonomicrometer consists of a pair of piezoelectric transducers, (a transmitter and a receiver), that are implanted into tissue, and connected to electronic circuitry. To measure the distance between the transducers, the transmitter is electrically energized to produce ultrasound. The resulting sound wave then propagates through the medium until it is detected by the receiver.
The transmitter is energized by a high voltage spike, or impulse function lasting under a microsecond. This causes the piezoelectric crystal to oscillate at its own characteristic resonant frequency. The envelope of the transmitter signal decays rapidly with time, usually producing a train of six or more cycles that propagate away from the transmitter through the aqueous medium. The sound energy also attenuates with every interface that it encounters.
The receiver is usually a piezoelectric crystal with similar characteristics to the transmitter crystal, that detects the sound energy and begins to vibrate. This vibration produces an electronic signal in the order of millivolts, that can be amplified by appropriate receiver circuitry.
The propagation velocity of ultrasound in aqueous media is well documented. The distance travelled by a pulse of ultrasound can therefore be measured simply by recording the time delay between the instant the sound is transmitted and when it is received.
Prior art sonomicrometers suffer from a number of shortcomings which limit their utility.
Firstly, conventional sonomicrometers use analog circuitry between transmit and receive signals (e.g. phase capacitative charging circuits). The voltage representing the measured distance is then output to a strip chart recorder in analog form. This data must then be digitized for computer analysis.
Secondly, conventional systems use analog potentiometers to adjust the inhibit time and the threshold voltage that triggers the receiver circuits. This often requires the use of an oscilloscope. Each time the system is used, these settings must be manually set and adjusted in order to tune the system. This can be time consuming and annoying. As a whole, the function of the system can not be changed. The repetition frequency is fixed, regardless of the number of channels used, and the system is therefore very limited in terms both of the distances that can be measured, and the temporal precision with which the system operates.
Thirdly, conventional ultrasound tracking systems feature pairs of transmitter and receiver crystals that are energized sequentially at fixed repetition rates. As such, prior art systems lack experimental flexibility. For example, before a pair of crystals is implanted, the user must decide each crystal's function; similarly, the user must determine which distances are to be measured by which crystal pair. This can be awkward because surgery often necessitates changes during the procedure. If either of the receiver or transmitter crystals malfunctions, the distance between them cannot be measured. Critical measurements can therefore be lost after a significant amount of effort is put into setting up the surgery.
Fourthly, conventional sonomicrometer systems measure only a straight line distance between any isolated pair of crystals. Three dimensional information is therefore impossible to acquire.
Even if multiple combinations of distances could somehow be linked together, the inherently analog nature of the data would necessitate the use of additional, very complex hardware.
Finally, conventional systems use discrete elements, such as threshold capacitors and potentiometers requiring large plug-in units to increase the number of channels. The systems are very large, usually two feet wide by 18" deep, and up to 12" high. Additional hardware such as strip chart recorders must be used for visualization and subsequent processing. This can be very awkward given the space constraints at busy research institutes and hospitals.